Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure of Granular Plugging Zone in Deep Fractured Reservoirs

*Xiaopeng Yan, Song Deng, Mingguo Peng, Yili Kang, Chengyuan Xu, Yong He, Danielle S. Tan, Jiangshuai Wang, Hongda Hao and Chaowei Li*

## **Abstract**

Fracture plugging zone with low strength is one of the key reasons for plugging failure in deep fractured reservoirs. Forming a high-strength plugging zone is a key engineering problem to be solved in wellbore strengthening. In this chapter, wellbore strengthening mechanisms of plugging zone for wellbore strengthening in deep fractured reservoirs are revealed from a relationship between mechanical structure and strength standpoint. Physical granular bridging materials dislocation and crushing under pressure fluctuation induce the strong force chains network failure, which leads to macroscale friction or shear failure of plugging zone. The main methods to improve microscale materials stability are to increase friction resistance, exert embedding effect, and strengthen bonding effect. Factors, which strengthen the meso-structure stability, include increasing shear strength and proportion of strong force chains. Key measures to strengthen the macrostructure stability of plugging zone are by improving its compactness, controlling its length, and ensuring the stability timeliness.

**Keywords:** lost circulation control, granular plugging zone, pressure containment, failure of the multi-scale structure, structure control, strengthening methods

## **1. Introduction**

Working fluid loss is a phenomenon that occurs frequently during the process of drilling and completion in fractured formations due to the low-pressure -bearing capacity of the formation [1–3]. Working fluid loss causes the most serious reservoir damage during drilling and completion in petroleum and geothermal resource. It is also one of the most complex engineering problems to solve in that it affects safe

and efficient drilling and oil and gas well productivity for a long time [4]. The maximum liquid column pressure that the wellbore can bear without fluid loss is called formation pressure-bearing capacity [5, 6]. Technologies to improve the formation pressure-bearing capacity (also called wellbore strengthening technologies) are often used to control the loss of the working fluid. The pressure-bearing capacity of the formation can be improved by adding wellbore strengthening materials into the working fluid, which then expands the safety density window of the formation. The DEA-13 experiment carried out by the association of drilling engineers between the mid-1980s to the early 1990s confirmed that adding solid particles into the drilling fluid can greatly improve the pressure-bearing capacity of the formation [7]. Since then, a lot of research has been done on the loss control theory and on characterizing formations that experience high losses and have low - pressure-bearing capacities. The temporary plugging theory and its related method have solved the loss problem of pore type and fracture pore type reservoirs [8, 9].

In fractured reservoirs, the development of natural fractures greatly complicates efforts to improve the pressure-bearing capacity of the formation. The root cause of working fluid loss in fractured reservoirs is the imbalance between the wellbore pressure and the formation pressure/stress. At present, methods to improve pressurebearing capacity of fractured reservoirs mainly include increasing fracture pressure, increasing fracture extension pressure, and plugging loss channel [10, 11]. Ways to improve the pressure-bearing capacity of the reservoir are closely related to the degree of fracture development within the reservoir. The reservoirs can be divided into the undeveloped fractured reservoir, non-leaky natural fracture reservoir, and leaky natural fracture reservoir based on the degree of fracture development. The corresponding working fluid loss types for these three reservoirs being induced fracture loss, fracture extension loss, and large and medium fracture loss [12]. For the induced fracture type loss reservoir, the mechanism that leads to fluid loss is the imbalance between the wellbore pressure field and the *in situ* stress field. The key to improving the pressure-bearing capacity is to form a plugging zone near the fracture opening, support the fracture opening, and improve the fracture pressure of the reservoir. This is called the stress cage and fracture closure stress method [13]. The root cause of fluid loss in the fracture extended loss reservoir is the imbalance between the wellbore pressure field and the stress field at the fracture tip. For this type of reservoir, a plugging zone is formed within the fracture, and the key to controlling its fluid loss is to isolate the fracture tip and improve the fracture extension pressure (called increasing the fracture extension pressure method) [14–16]. In large and medium fracture type reservoirs, the imbalance between the wellbore pressure field and the formation pressure field is the main fluid loss mechanism. To improve the pressure-bearing capacity of this type of reservoir, it is necessary to add lost circulation materials (LCMs) to the working fluid to plug the fractures. The plugging zone that is formed within the fracture can block the transmission between wellbore pressure and formation pressure (called the plugging the loss channel method) [17–19]. Therefore, the essence of improving the pressure-bearing capacity of the reservoir is to form an artificial high-pressure-bearing plugging zone around the well. For fractured reservoirs, the key to improving the pressure-bearing capacity of the plugging zone is by improving the pressure-bearing capacity of the reservoir.

The relationship between structure and performance is continuously researched [20–22]. The structural stability of the plugging zone is the key factor that controls fracture reservoir fluid loss. The structural analysis of the plugging zone has become

## *Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure… DOI: http://dx.doi.org/10.5772/intechopen.112511*

important for the theory and technological development of lost circulation prevention and mitigation. Kang et al. introduced the particle material mechanics theory to study the plugging effect of fractures, pointing out that the plugging zone is a granular material system composed of LCMs. The structure of the plugging zone has multi-scale characteristics. The strength of the force chain in the mesoscale is closely related to the stability of the macroscale plugging zone. Xu et al. constructed the failure mode of the plugging zone under a high temperature, high pressure, and high-stress environment, established the strength model of the plugging zone and extracted the key performance parameters of LCMs [23]. Based on the theory of system science and mutation, She et al. proposed a mathematical model to describe the failure process of the fracture plugging system [24]. Considering the stress analysis of the microstructure of the plugging zone, Qiu et al. discussed the failure modes of the plugging zone such as crushing failure and seepage loss failure. Yan et al. defined the dominant force types among LCMs, extracted multi-scale structural parameters of plugging zone, and introduced a photoelastic experiment to characterize the force chain network [25].

Granular matter is called the fourth form of matter. Research on its multi-physical mechanisms and multi-scale structural stability has become a hot spot at the frontier of particle material research [26–28]. Although some progress has been made in the research on the structural characterization of the plugging zone, the multi-scale structural failure mechanism of the plugging zone is not yet clear. In this chapter, recent advances in wellbore strengthening mechanisms are presented from a relationship between mechanical structure and strength standpoint.

## **2. Multi-scale failure mechanisms of plugging zone**

## **2.1 Macroscale structural failure of plugging zone**

On the macro scale, the structural failure of plugging zone can be divided into friction failure and shear failure [23]. The reason why the plugging zone can exist within the fracture is that the friction force is formed by the contact between plugging zone and the fracture surface. After plugging zone has formed, it is affected by the combined action of the wellbore pressure and the fracture pressure along the direction of the fracture length (**Figure 1a** and **b**). When the friction between plugging zone and fracture surface is less than the resultant force of wellbore pressure and pressure in the fracture, plugging zone will slide to the fracture interior. The structure will gradually disintegrate, resulting in friction failure (**Figure 1c**). The plugging zone shear failure refers to the failure in which plugging zone is first destroyed at a weak point under the action of the pressure difference between wellbore pressure and fracture pressure (**Figure 1d**). The nonuniformity of the development of micro convex bodies on the fracture surface means that the friction between the plugging zone and the fracture surface is not equal everywhere [23]. The parts with low friction between the plugging zone and the fracture surface are more likely to be damaged. Most importantly, because the formation of plugging zone is a random process, the structure of the plugging zone has a certain heterogeneity, and therefore contains structural weaknesses in the plugging zone. When pressure is applied to the plugging zone, structural failure will first occur at these weak points and then induce shear failure.

#### **Figure 1.**

*Schematic diagram of the multi-scale structural failure of plugging zone.*

## **2.2 Mesoscale structural failure of plugging zone**

Compared with an ordinary solid, the plugging zone is a discrete granular material system, which shows strong discontinuities and contact energy dissipation. This causes even more complex macro-mechanical behavior [29, 30]. Under an external load, the plugging zone will produce a certain mechanical response. The particles in the plugging zone will be squeezed and come into contact with each other, forming a quasi-linear chain network structure, called a force chain network (**Figure 1e**). The force chain network is the transmission path of the indirect contact force of LCM along with the particle contact network. It provides the physical pathway for the transmission of the external load and directly reflects the internal stress of the discrete particle material system. The force chain network can be divided into strong and weak areas. The number of force chains is small, but it carries the main external load of the system. The distribution of the weak force chain and the surrounding strong force chains help to maintain the stability of the system. The photoelastic experiment method can effectively characterize the evolution process of the meso force chain network of the plugging zone during a pressure-bearing process. As shown in **Figure 1f**, after the vertical load and shear load are applied to the plugging zone simultaneously, several bright force chains appear along the shear direction begin to appear and they connect to the force chains in the vertical direction to form a strong force chain network. When the shear load continues to

## *Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure… DOI: http://dx.doi.org/10.5772/intechopen.112511*

increase to the bearing limit of the plugging zone, the strong force chain network inside the plugging zone breaks (**Figure 1g**). The results of the photoelastic experiment show that the pressure failure of the plugging zone results in a fracture of the strong force chain network at mesoscale.

## **2.3 Microscale structural failure of plugging zone**

After the LCMs are retained in the fracture, it forms a plugging zone under the combined action of the wellbore pressure difference, the *in situ* stress, and the contact force of adjacent particles. The LCMs are squeezed and come into contact with each other to maintain a relatively stable state (**Figure 1i**). The bridging materials in the plugging zone bear the main external load. When the pressure fluctuates, some bridging materials in the plugging zone may lose their mechanical balance, and the particles may move together with each other. The dislocation modes of the micro LCMs include particle friction sliding (**Figure 1j**) and particle rotation (**Figure 1k**). In practice, the dislocations of LCMs include both frictional sliding and turning, such as particle climbing (**Figure 1l**). After plugging zone is formed, a strong force chain network is caused by the sliding or rotation friction of micro bridging particles, which leads to the friction and shear failure of the macro plugging zone.

## **3. Microstructure strengthening of plugging zone**

## **3.1 Improving friction resistance of LCMs**

Friction sliding of LCMs is one of the main reasons for the dislocation of LCMs on a microscale. An important means to strengthen the microstructure stability of LCMs is by improving friction resistance between LCMs. The geometry and elasticity of LCMs are the main controllable factors. The results of the friction coefficient test show that the lower the roundness of the LCM, the higher the friction coefficient. The friction coefficient between the LCMs can be improved by adding elastic particles to a single rigid particle (**Figure 2**). By optimizing the irregular rigid bridging materials, the meshing degree between the LCMs can be improved, and the friction coefficient between the LCMs can be improved. At the same time, the introduction of highly elastic and highly expansive filling materials can increase the contact area between the LCMs, and thereby increase the effective stress on the LCMs, to improve the friction resistance of the LCMs.

## **3.2 Give full play to the embedding effect of the high-strength rigid particles**

For deep fractured reservoirs, particle breakage can induce microstructure failure of the LCMs. Under conditions of high deep fracture closure stress, LCMs can easily be compressed and broken. As reported in [25], millimeter-grade calcium carbonate particles show significant particle size degradation after 30 minutes under a pressure of 25 MPa of fracture closure stress, while the D90 degradation rate of its particle size distribution reaches 17.7%. After the LCMs are broken, the space around the neighboring particles increases, which makes it easier for the particles to move. High fracture closure stresses can induce compression and breakage the LCMs. However, these stresses can also act as a favorable factor in cases, where they provide a power source for the proper embedding of LCMs. This is achieved by optimizing the strength and toughness of rigid bridging particles, which enhances the stability of the LCMs (**Figure 3**).

#### **Figure 2.**

*Friction coefficient curve of single high sphericity rigid particles and a combination of rigid, elastic, and fiber LCMs (Modified from [27]).*

#### **Figure 3.**

*Crushing rate under 30 MPa effective stress of rigid granular materials commonly used in drilling sites (Modified from [26]).*

*Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure… DOI: http://dx.doi.org/10.5772/intechopen.112511*

### **3.3 Introduction of bonding to strengthen the stability of LCMs**

Two types of forces exist between the LCM particles: contact forces, and noncontact forces. Noncontact forces include electrostatic force, van der Waals force, and liquid bridge force. The electrostatic force between particles can be ignored because the particles are usually inert and electrically neutral. The particle size is generally between hundreds of microns and several millimeters, and therefore the van der Waals forces between the particles are often small. The liquid bridge force is mainly derived from the surface tension at the solid–liquid–gas interface. For the drilling fluid system or plugging slurry system, generally, there are only two phases: liquid phase and solid phase. While the solid–liquid–gas interface is negligible, so the interparticle liquid bridge force effect in plugging zone can also be considered nonexistent. The contact force between particles is exerted by the fracture closure stress. The fracture closure stress that acts on the LCMs can often reach tens of MPa. The contact force between these particles, which plays a major role at this depth, is far greater than the noncontact forces between the particles [25]. To further enhance the structural stability of the LCMs, a soluble bonding material can be added into the granular plugging formula. This not only address the need for plugging removal before putting into operation but also play a bonding role that can further solidify the LCMs and improve the stability of the LCMs. The fracture plugging experiment shows that the plugging zone pressure-bearing capacity formed by a single type of spherical, sheet, and fiber LCMs is relatively low. The pressure-bearing capacity of a single fiber material is only 1.4 MPa, while the pressure-bearing capacity of the plugging zone formed by fiber combined with polypropylene binder can be increased to 6.9 MPa [31].

## **4. Meso-structure strengthening of plugging zone**

### **4.1 Increase the shear strength of the strong force chain**

The mesoscopic failure of the plugging zone can be attributed to failure of the strong force chains network in the plugging zone. The strong force chains network is formed by the connection of multiple strong force chains, but the shear strength of a single chain directly controls the pressure stability of the strong force chains network. According to the mechanics of the granular matter, the shear strength of a single force chain is equal to the product of the friction coefficient between the particles and the contact stress between the particles. The contact stress between the particles depends on the stiffness, deformation, and contact area of the particles. In a weak force chain, the contact degree between particles is low and the deformation of the particles is small, which makes it difficult to bear a higher shear load. Conversely, in strong force chains, the degree of contact between the particles is high, the external load borne by particles is large, and the deformation of particles is relatively large. When the action line of the particle contact force is within the range of particle friction angle, the particles in the strong force chains are in a relatively stable state. The higher the friction coefficient of the particle surface, the greater the stiffness and the greater the degree of extrusion deformation, and the higher shear strength of strong force chains. By optimizing the performance parameters of the LCMs, the strong force chain LCMs with a high shear strength can be selected to enhance the stability of the meso-structure of the plugging zone (**Figure 4**).

**Figure 4.**

*The shear strength of the strong force chain of plugging zone formed several typical LCMs.*

## **4.2 Increase the number of strong force chains**

The bearing stability of the strong force chain network of the plugging zone is determined by both the shear strength of a single strong force chain and the number of strong force chains. The photoelastic experiment results show that strong force chains proportion can be increased by selecting rigid LCMs with low sphericity and by adding more elastic LCMs. The strong force chains exist in a dense granular material system. By optimizing the concentration and proportion of the bridging and filling particles, and by improving the coordination of the LCMs, the probability of forming strong force chains can be improved. The type, concentration, and ratio of LCMs can be optimized to increase the proportion of the strong force chains, which can increase the pressure-bearing stability of the meso-structure in the plugging zone. As shown in **Figure 5**, compared with the single round granule plugging zone, the development of the force chain network in the plugging zone is improved after adding triangular and square particles. The addition of angular particles to round particles makes the force chain network more diversified, increases the proportion of strong force chain, and improves the stability of the plugging zone.

## **5. Macro-structure strengthening of plugging zone**

### **5.1 Ensure the timeliness of plugging zone stability**

As the exploration and development of oil and gas resources reach ever greater depths, repeated fluid loss occurs frequently [32]. For example, the Tarim Basin has a reservoir depth of 7700 m, a reservoir temperature of 168°C, and an average total salinity of 210,000 mg/L for the formation water. The minimum horizontal principal *Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure… DOI: http://dx.doi.org/10.5772/intechopen.112511*

#### **Figure 5.**

*Photoelastic images of the plugging zone under an external load. (a) Single round particles, (b) combination of round and triangular particles, and (c) combination of round and square particles.*

stress is 167 MPa, while the pore pressure is 120 MPa. In this example, the force exerted on the plugging zone was 47 MPa. During the drilling of the reservoir section in the Tarim basin, multiple fluid losses occurred, with the loss amount ranging from 24.5 to 573.0 m3 , with an average loss amount as high as 164.7 m3 . The long-term stability of plugging zone under a high-temperature, high-stress, high-salinity environment has become one of the key technical challenges in ensuring deep fractured reservoir loss control. Based on conventional particle size distributions, acid solubility, and other evaluation indexes of LCMs, it is necessary to consider the temperature resistance, salt resistance, and compression resistance of LCMs. Optimizing the temperature-resistant, salt-resistant, and high-stress resistant LCMs is important to ensure stable and timely plugging of the fracture zone.

#### **5.2 Improve the compactness of plugging zone**

Compactness is one of the key performance parameters to strengthen the macrostructure of the plugging zone. When the density of the plugging zone is poor, the particle system is loose and its structural stability poor. The density of the plugging zone is closely related to the concentration, gradation, and type of LCM. For a certain fracture width, particle interaction and the formation of a tight plugging zone are enhanced with an increase in LCM concentration. Compared with the single-particle size LCM, the LCM with a specific particle size distribution improves the plugging zone density. For the millimeter-sized loss fractures, which are difficult to control, the optimal combination of LCMs is a rigid granular material, elastic granular material, and fiber material. Rigid particles play a major role in bridging and form the main framework of the plugging zone. Elastic particles fill in the gaps between the rigid particles, which not only reduces the porosity of the plugging zone but also improves the toughness of the plugging zone. Fiber material with a high aspect ratio and good flexibility can further improve the fracture plugging compactness. By optimizing the performance matching of LCMs, the density of the plugging zone can be improved.

## **5.3 Control the plugging length of the plugging zone**

The structural pressure-bearing stability of the plugging zone is not only controlled by LCMs performance parameters and the plugging slurry formula but is also closely related to the plugging technology. Bridging plugging is the most commonly used loss control technology in fractured reservoirs. The field practice of bridging plugging shows that controlling the squeezing pressure can gradually increase the length of the fracture plugging zone. The optional squeezing strategy is intermittent squeezing in small amounts to squeeze as much of the plugging slurry into the formation as possible. With the increase of the length of the fracture plugging zone, the stability of the fracture plugging zone is enhanced. By using long core fracture plugging simulation experiments, the bridging plugging technology can be optimized in terms of squeezing pressure, single squeezing volume, and squeezing times. And the favorable conditions for the formation of fracture plugging zone with high-pressure stability can be created.

## **6. Conclusion**

The following conclusions are obtained:


## **Acknowledgements**

The authors gratefully acknowledge the Fourth Batch of Leading Innovative Talent's Introduction and Cultivation Projects of Changzhou (No. CQ20220087), the *Constructing a Tough Shield around the Wellbore by Stabilizing the Multi-Scale Structure… DOI: http://dx.doi.org/10.5772/intechopen.112511*

Scientific Research Foundation for Innovative Talents Introduction of Changzhou University (No. ZMF23020041). The authors would also like to acknowledge the useful suggestions from Prof. Xiangyu Shang, Dr. Jingyi Zhang and Dr. Chong Lin.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Xiaopeng Yan1 \*, Song Deng1 , Mingguo Peng1 , Yili Kang2 , Chengyuan Xu2 , Yong He3 , Danielle S. Tan4 , Jiangshuai Wang1 , Hongda Hao1 and Chaowei Li1

1 School of Petroleum and Natural Gas Engineering, Changzhou University, Changzhou, China

2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China

3 TLM Oilfield Company, CNPC, Korla, Xinjiang, China

4 Department of Mechanical Engineering, National University of Singapore, Singapore

\*Address all correspondence to: lcm\_yxp2017@126.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 7**

## Cement Sheath Integrity in Oil and Gas Wells

*Chenwang Gu, Yongcun Feng and Xiaorong Li*

## **Abstract**

The cement sheath is a crucial component of the wellbore system, responsible for maintaining structural integrity, and preventing leakage. Over the life cycle of oil and gas wells, load changes can lead to various cement failure modes, such as disking, radial cracks, and debonding fractures. It is vital to locate and evaluate cement sheath failure in the wellbore. This chapter aims to comprehensively and systematically describe recent advances in cement sheath integrity prediction, control, and monitoring techniques. Firstly, we list the underlying reasons for the cement sheath failure. Then, we extensively discuss current advances in cement sheath integrity. Finally, wellbore integrity control and monitoring techniques are also discussed. This chapter serves as a valuable reference for both scientific research and engineering applications of cement sheath integrity in oil and gas wells.

**Keywords:** cement sheath, cement failure, interface debonding, integrity prediction, monitoring

## **1. Introduction**

Wellbore integrity refers to the implementation of a range of technological, operational, and organizational management measures aimed at minimizing the risk of uncontrolled leakage of formation fluids throughout the entire life cycle of oil and gas wells [1]. The primary objective is to establish effective wellbore barriers at each wellbore stage [2]. The API RP 90 defines the wellbore barrier as an "envelope of one or several dependent barrier elements preventing fluids or gases from flowing unintentionally from the formation into another formation or to the surface," as shown in **Figure 1** [3]. The failure of wellbore integrity in oil and gas production can result in various issues, including lost circulation, casing instability, formation pollution, and environmental pollution. This significantly impacts the safety and efficiency of oil and gas exploration and production activities. Therefore, safeguarding wellbore integrity is an essential task in the oil and gas field development [4].

The cement sheath, as a pivotal component in the cementing process of oil and gas wells, plays a significant role in establishing a stable and sealed structure between the casing and the formation. Referring to "API RP 90 Annular Casing Pressure Management for Offshore Wells" [5] and "NORSOK STANDARD D-010 Well integrity in drilling and well operations," [6] cement sheath integrity is defined as the scientific

**Figure 1.** *General principles of well barriers [3].*

design of the cement slurry system and implementation of effective technical measures to prevent mechanical integrity and hydraulic sealing failures of the cement sheath. This approach helps minimize the risk of uncontrolled flow of formation fluids throughout the entire life cycle of the borehole and ensures the safe drilling and production of oil and gas wells.

The quality and performance of the cement sheath are directly related to the production and recovery of oil and gas wells. Furthermore, it significantly impacts the environmental and social benefits associated with oil and gas production. Statistics show that as many as 6692 wells in the OCS area of the Gulf of Mexico in the United States have sustained casing pressure (SCP), accounting for 43% of the total number of wells [7]. There are 166 production wells in the Fuling shale gas field in Sichuan, China, and 79.524% of the wells have SCP problems [8]. Therefore, the research and optimization of the cement sheath is of great significance.

## **2. Reasons for cement failure**

The cement sheath is subjected to various complex loads downhole, and the failure of sealing integrity can be divided into the following three categories: shear failure, tensile failure, and interface debonding (**Figure 2**) [9]. The main reasons for the cement sheath failure are mainly described below.

#### **2.1 Poor cementing quality**

Cementing quality is crucial for ensuring both safe and efficient drilling to the designed well depth and the secure production of oil and gas. Several factors influence *Cement Sheath Integrity in Oil and Gas Wells DOI: http://dx.doi.org/10.5772/intechopen.113052*

**Figure 2.** *Schematic of cement sheath failure [9].*

cementing quality, including the geological characteristics of oil and gas reservoirs, geometric conditions of boreholes, properties of formation fluids, well completion production methods, requirements for reservoir protection and stimulation, technical effects of previous operations, performance of cement slurry, and construction technology [10].

Inhomogeneous cement mixtures can lead to uneven density, hardness, and strength of the cement sheath, thereby affecting the cement sheath integrity [11]. For example, excess water or additives in the cement mixture may result in cracking of the cement sheath. The improper rheological properties of the cement slurry can result in a substandard cement sheath quality. For instance, excessively strong or weak fluidity may lead to a failure in the integrity of the cement sheath. Incorrect operation during the cementing process, such as excessively high or low cementing pressure, overly fast or slow cementing rate, extremely high or low cementing slurry temperature, etc., may lead to the failure of the cement sheath integrity [12]. Therefore, to ensure the quality of the cement sheath, it is necessary to adopt appropriate operational measures and monitoring methods during the cementing process to ensure the stability and reliability of the cementing quality.

### **2.2 Temperature and pressure change**

Several special operations in oil and gas wells, such as hydraulic fracturing, CO2 injection, and gas storage cycle injection and production, can significantly change the pressure and temperature of the wellbore. The change in temperature and pressure will affect the stress distribution in the cement sheath, which may cause the seal integrity failure of the cement sheath [13–16]. Notably, drilling, fracturing

stimulation, and production process induce pressure changes in the wellbore, and excessive stress will lead to the failure of the cement sheath structure. Furthermore, the injection of fracturing fluid into the formation, influx of fluids from the formation, and steam injection cause temperature fluctuations within the wellbore. Consequently, it is imperative to investigate the influence of pressure and temperature changes on the integrity of the cement sheath.

#### **2.3 Cement shrinkage**

During the hardening of cementing, the cement slurry system is always accompanied by volume changes in liquid, plastic, and solid states, that is. "volume shrinkage." This shrinkage occurs due to a chemical reaction between the cement clinker and water, leading to an overall reduction in the volume of the cement slurry [17, 18]. The absolute volume shrinkage of Portland cement slurries ranges from 2.3 to 5.1%. The formation of a gelled structure from the solidification of the cement slurry restricts the downward transmission of gravity and causes a drop in effective liquid column pressure. Consequently, the formation fluid pressure surpasses this weakened pressure, resulting in fluid entering the annulus and leading to channeling.

The volume shrinkage of cement can lead to cracks in the cement sheath. The main reason is that the radial tensile stress of the cement sheath gradually increases during the shrinkage process. When the tensile stress exceeds the tensile strength of the cement, the cement sheath will produce radial cracks, as shown in **Figure 3** [19]. The volume shrinkage of cement can also lead to the development of micro-annulus between the cement sheath and the formation and the casing, resulting in the development of channels for oil, gas, and water flow. Therefore, controlling the shrinkage of cement volume during and after cement slurry curing is extremely important to guarantee cementing quality.

**Figure 3.** *Distribution of radial cracks in the cement sheath at different stages [19].*

## **2.4 Chemical degradation**

The cement sheath is essentially a Portland cement-based material, which is extremely vulnerable to acidic medium corrosion, resulting in alteration of material composition, structural damage, and functional degradation or loss [20]. The corrosion of cement stone by CO2 is a complex and prolonged physical and chemical process [21].

The CO2 corrosion of the cement sheath includes the following three processes. The carbonic acid is generated through the dissolution of carbon dioxide in the formation water. Next, this carbonic acid reacts with calcium hydroxide and calcium silicate hydrate in the cement matrix, forming microspherical calcium carbonate crystals. These crystals lead to the formation of compounds known as Ca(OH)2- CaCO3 and C-S-H-SiO2-CaCO3, respectively. These compounds contribute to an increase in both the equivalent elastic modulus and the equivalent hardness of the cement matrix. Additionally, they also lead to a reduction in both the porosity and permeability of the cement matrix. The specific processes can be observed in formulas [22, 23].

$$\text{CO}\_2 + \text{H}\_2\text{O} \rightarrow \text{H}\_2\text{CO}\_3\tag{1}$$

$$\text{Ca(OH)}\_{2} + \text{H}\_{2}\text{CO}\_{3} \rightarrow \text{CaCO}\_{3} + \text{H}\_{2}\text{O} \tag{2}$$

$$\text{CSH} + \text{H}\_2\text{CO}\_3 \rightarrow \text{CaCO}\_3 + \text{SiO}\_2 + \text{H}\_2\text{O} \tag{3}$$

As carbon dioxide continues to dissolve in the formation water, the carbonization process continues, and CaCO3 is formed into Ca(HCO3)2. Ca(HCO3)2 is soluble in water and diffuses into formation water, and this phenomenon is known as the leaching of calcium ions.

$$\rm{CaO}\_2 + \rm{H}\_2\rm{O} + \rm{CaCO}\_3 \rightarrow \rm{Ca(HCO}\_3\rm{)}\_2\tag{4}$$

Due to the dissolution of Ca(OH)2 and CaCO3, the pH value of the cement matrix increases. As a result, the hydrated calcium silicate gel undergoes transformation into amorphous silica gel, disrupting the internal bonding conditions of the cement matrix and increasing its porosity. Meanwhile, the solid phase particles in the cement matrix dissolve and diffuse into the formation water. This process facilitates the

**Figure 4.** *Interpretation of experimental result of CO2 reaction with cement [24].*

interconnection of pores and cracks in the cement matrix, creating a pathway for corrosive fluids to penetrate the interior and initiate corrosion within the matrix (**Figure 4**).

The interpretation of experimental results of the reaction between CO2 and cement is shown in **Figure 4** [24].

#### **2.5 Perforating operation**

In the development of unconventional oil and gas resources, the perforation completion method is often employed to establish the connection between the formation and the wellbore [25, 26]. During the perforation explosion, high-temperature/highpressure metal jets are generated, which rapidly penetrate the casing, cement sheath, and formation at speeds ranging from 3000 to 8000 m/s and 10–30 GPa pressures. This process occurs within a very short period. The completion of perforation often leads to cracks in the cement sheath. Furthermore, the high pressure experienced during fracturing may cause the cement sheath to debond from the casing, resulting in channeling between oil layers, groundwater layers, and gas layers [27]. For example, Daqing Oilfield conducted ultrasonic testing before and after the perforation operation of 19 wells in a block and found that the cement sheaths of many wells had local debonding and cracking problems. Therefore, clarifying the cause of cement sheath rupture in perforated well completion is the key to improving cementing quality and successfully developing unconventional oil and gas.

## **3. Cement sheath failure research**

Cement sheath is a crucial component of oil and gas wells. Issues in cementing quality or complex loads from downhole operations can lead to yielding and damage of the cement sheath, which compromises its sealing ability and deteriorates casing stress. The integrity of the cement sheath is directly linked to the safe extraction of oil and gas from the wellbore and ensuring environmental protection. Therefore, it is crucial and imperative to analyze the integrity of the cement sheath.

#### **3.1 Cement failure**

Due to various factors such as harsh downhole conditions, temperature, and pressure variations, the cement sheath is subjected to different levels of damage and failure. This further increases the risk of wellbore fluid leakage, posing potential threats to the environment, and personnel safety. The failure mechanisms of cement sheath are complex and diverse, primarily including tensile failure and shear failure.

Currently, some progress has been made in the research on the failure mechanisms of cement sheath, but there are still some limitations. The existing studies on the failure of the cement sheath mainly use the ideal constitutive model to describe the mechanical behavior of cement. However, these idealized models are difficult to accurately reflect the mechanical behavior of cement in the actual formation environment. Consequently, this could lead to inaccurate predictions of cement sheath failure behavior. In real downhole environments, the cement sheath is subjected to various mechanical and chemical effects, such as temperature changes, stress changes, formation fluid invasion, etc. Nevertheless, current studies often simplify these factors into a single influencing factor, failing to accurately capture the complex mechanical

#### *Cement Sheath Integrity in Oil and Gas Wells DOI: http://dx.doi.org/10.5772/intechopen.113052*

response of the cement sheath in the actual formation environment. Therefore, future research should focus on enhancing the observation and experimental research of the cement sheath in the actual formation environment. This approach could obtain more accurate mechanical behavior data, and develop a constitutive model that can reflect the cement in the actual formation environment. Additionally, the influence of the actual formation environment on the cement sheath failure should be comprehensively analyzed in combination with field observation data and numerical simulation methods. Such an integrated approach would significantly improve our understanding and predictive capabilities regarding this issue. By bridging the gap between theoretical models and real-world conditions, advancements in this field can contribute to safer and more efficient oil and gas well operations.

### **3.2 Cement interface debonding**

According to the cause of interface debonding, interface debonding can be divided into three types. The first is that under the action of cyclic temperature and pressure, the cement sheath produces cumulative plastic strain, which causes the casing and cement sheath to deform uncoordinatedly, resulting in micro-annulus. The second is that the interface is peeled off under the action of formation fluid driving pressure due to the poor cementing quality. The third is that the volume of the cement sheath shrinks continuously during the process of cement sheath solidification and continuous loss of water. When the radial stress at the interface becomes tensile stress and overcomes the tensile strength of the interface bonding, it may also cause interface debonding.

The current research is mainly based on experiments and numerical simulation methods, lacking the observation data of actual well sites. Furthermore, most existing studies are confined to ideal conditions and fail to deeply explore the impact of complex formation conditions and operational changes that occur in actual well environments. As a result, there is a need for more comprehensive investigations in realistic well conditions. Additionally, the understanding of the mechanism and influencing factors of interface failure under thermal-hydrological-mechanical– chemical coupling remains limited. The fracture behavior of the interface and the crack propagation mechanism need to be further studied.

## **4. Cement integrity control technology**

The research and development of new cement aims to overcome the limitations of traditional cementing by enhancing various properties, including higher strength, improved corrosion resistance, better self-healing capabilities, superior expansion, and increased toughness.

#### **4.1 Anticorrosive cement**

The presence of acidic gases, such as CO2 and H2S, can lead to the corrosion of cement when exposed to suitable levels of humidity and pressure. This corrosive action has several negative effects on the cement, including a reduction in its alkalinity, a decrease in the compressive strength of the cement sheath, an increase in permeability, and ultimately, a shortened production life for oil and gas wells.

Consequently, it becomes crucial to conduct research on corrosion-resistant cementing materials to mitigate these detrimental impacts [28].

Currently, there are two main methods to solve the problem of cement corrosion caused by acidic medium: (1) Reduce the content of alkaline substances in cement. By adding a type of external filler that reacts with Ca(OH)2 inside the cement, the content of alkaline substances in the cement is reduced. (2) Limit the seepage process to inhibit the corrosion chemical reaction. The pore structure of cement determines the speed at which the corrosive medium penetrates into the cement. To counteract this, certain admixtures can be incorporated to enhance the compactness of the cement. Polymer materials, such as latex, asphalt, and resin, are particularly valuable due to their small particle size, deformability, and corrosion resistance. Therefore, these materials are widely used in the preparation of corrosion-resistant cement slurry systems.

#### **4.2 Self-healing cement**

Self-healing cement technology refers to the ability of cement to undergo partial or complete self-repair after sustaining damage. This repair process is facilitated by external factors, as well as inherent conditions, which trigger a series of physical, chemical, and biological reactions. Through these reactions, the cement releases filling materials or generate new substances, enabling the restoration of its integrity [28]. Based on different self-healing principles, this technology can be categorized into four main types: original autonomous self-healing technology, external stimulusresponsive self-healing technology, microcapsule/fiber rupture filling self-healing technology, and microbial metabolite self-healing technology.

Since the self-healing cement is used in the special environment of downhole, the self-healing materials used in the cement slurry must meet the following requirements: (1) The cement sheath formed after the addition of self-healing materials should have mechanical properties that meet the requirements for interlayer isolation. (2) It can make the wellbore withstand the alternating stress and downhole temperature gradient changes during injection and production and meet the technical requirements for long-term hydraulic sealing in the downhole. (3) The performance of the cement sheath after adding the self-healing material should be stable for a long time, and the self-healing material should have good durability. After a certain period of time, the self-healing material can still produce a self-healing effect on micro-cracks [29, 30].

#### **4.3 Expansive cement**

Expansion cement for well cementing is a particular type of cement that can expand during the cementing process. It is typically composed of crystal-based expansion materials or gas-releasing expansion agents. Crystal-based expansion materials include calcium aluminate-based crystals and alkaline-earth metal oxidebased materials. The main expansion driving force for calcium aluminate-based materials is calcium aluminate, which includes components, such as sulfates and aluminates. Alkaline earth metal oxide-based materials include CaO expansion agents and MgO expansion agents [31]. Expansion cement plays a significant role in oil and gas extraction by improving cementing effectiveness, reducing wellbore leakage rates, and ensuring the safety and efficiency of the extraction process.

## **4.4 High toughness cement**

The incorporation of toughening materials into the cement slurry results in enhanced impact resistance and compressive performance of the cement annulus, while also inhibiting the development of surface cracks to a certain extent. When external forces are applied to the cement sheath, the toughening materials significantly improve the load-bearing capacity of the cement matrix. Initially, stress primarily acts on the cement matrix, leading to the appearance of surface cracks. However, the crisscrossing fibers within these cracks become critical load-bearing elements, strengthening the material's ability to handle stress. As stress increases further and exceeds the load-bearing capacity of the fibers, they fracture and detach from the cement matrix. Relevant research shows that the traditional cement sheath will be damaged after 2–10 cycles of cyclic stress. In contrast, cement sheaths with toughening materials can withstand tens of thousands of cyclic stress cycles. In summary, the addition of an appropriate amount of fiber materials during the preparation of cement slurry can influence the microstructure of cement sheath, enhance their stress–strain characteristics, and improve their impact resistance.

## **5. Cement failure monitoring**

On-site cement sheath integrity monitoring is a crucial task in petroleum engineering. Its purpose is to assess the quality and integrity of the cement sheath in the wellbore, ensuring the safe operation of oil and gas wells and environmental protection.

Before beginning well test and production operations, cement-casing system integrity is assessed primarily using cement bond logs and variable-density logs (CBL/ VDL). The CBL/VDL tool's working concept is depicted in **Figure 5**. Signal

**Figure 5.** *The operation principle of CBL/VDL tool [32].*

**Figure 6.**

*A schematic representation of the DAS sensing unit [33].*

attenuation, which has a high correlation to the quality of the cement bond at the cement-casing interface, is recorded by the receiver as a result of changes in the wellbore system. As the signal moves through the wellbore system, the VDL captures the acoustic waveform that reaches the distant receiver. The degree of the bonding quality at the cement formation is shown by the signal attenuation. However, it should be noted that the CBL signal can be affected by various factors, such as the rheological characteristics of the cement slurry, the eccentricity of the cement sheath, the modulus of the cement and casing, and the centralization of the measurement tools. These influences may pose challenges in accurately representing the integrity of a well using typical acoustic logging techniques.

Distributed acoustic sensing (DAS) monitoring technology is an effective method for monitoring the integrity of cement sheaths in downhole environments. This innovative approach utilizes optical fiber sensors to continuously assess various parameters, such as acoustic waves, temperature, and pressure, enabling real-time detection of cement sheath integrity, cracks, and fracturing. **Figure 6** shows schematic diagrams of the DAS system. To implement DAS technology, optical fiber serves as a sensor and is excited and detected using lasers and detectors. By analyzing the reflected light signals in the optical fiber, changes in the downhole environment's parameters can be obtained and assessed. Consequently, the condition of the cement sheath, the presence of cracks, and fracturing can be accurately judged. Compared to traditional monitoring technologies, DAS offers distinct advantages, including a wide monitoring range, high resolution, and real-time solid performance. As a result, DAS has found extensive application in monitoring cement sheath integrity during cementing operations.

## **6. Conclusion**

The cement sheath integrity plays a critical role in the long-term safe and stable operation of the wellbore. This chapter systematically reviews the influencing factors of cement sheath integrity, prediction and evaluation model, cementing quality control technology, and cement sheath integrity monitoring technology.

The failure modes of the cement sheath primarily consist of tensile failure, shear failure, and interface debonding. These failures can be attributed to various factors, including cementing quality problems, temperature and pressure effects, cement shrinkage, corrosion erosion, and other complex factors. Understanding the failure mechanisms of the cement sheath accurately holds significant importance in improving cementing quality. The cement sheath and interface failure model considering

### *Cement Sheath Integrity in Oil and Gas Wells DOI: http://dx.doi.org/10.5772/intechopen.113052*

multi-field coupling can reasonably predict the cement sheath damage process under complex loading conditions. The development of novel anti-corrosion, self-healing and other special cementing, and the optimization of construction technology are effective ways to ensure the long-term integrity of cement sheath. The application of monitoring technologies, such as CBL, VDL, USI, DAS, and other means, can realize the monitoring and evaluation of the cement sheath integrity.

The future research on the integrity of cement sheath in well cementing lies in establishing an integrated system for well cementing technology that encompasses multiple-field coupling prediction models, intelligent process monitoring, and new types of adaptive cementing materials.

## **Author details**

Chenwang Gu1 , Yongcun Feng<sup>1</sup> \* and Xiaorong Li<sup>2</sup>

1 College of Petroleum Engineering, China University of Petroleum, Beijing, China

2 College of Safety and Ocean Engineering, China University of Petroleum, Beijing, China

\*Address all correspondence to: yfeng@cup.edu.cn

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 8**
